I've always thought that in order to become a project manager it was necessary a period as programmer (as it was a sort of promotion).
But according to what I read here it seems like a lot of pm have no idea how to/how long it takes to develop software... Am I wrong or what?

Nope, project managers don’t (in my experience) have any idea about the programming side of things, usually a promotion from the business / operations side of things.

There probably are good pm’s to but they just rarely figure in rants ;)

I'd say that PMs knowledge of development will range from 0-1. Everyone is on the scale somewhere, just at different points.

I feel like it's always been an argument for/against from many years ago but now I feel like it matters less if people are moving away from running 12+month waterfall projects and instead towards incremental delivery, hopefully estimated and driven by the actual team members themselves.

The places that still have PMs managing tech teams.. I'd hope they at least pick up some knowledge and compentency and aren't a complete muppet - i.e don't make up their own deadlines or estimates without consulting the team members.